Say to Aaron: It was inappropriate for the dust to be smitten through Moses since it had protected him when he slew the Egyptian and had hidden him in the sand. [Therefore,] it was smitten through Aaron [instead]. — [from Tanchuma, Va’era 14, Exod. Rabbah 10:7]

13They did so, and Aaron stretched forth his hand with his staff and struck the dust of the earth, and the lice were upon man and beast; all the dust of the earth became lice throughout the entire land of Egypt.

It is the finger of God: This plague is not through sorcery; it is from the Omnipresent. — [from Exod. Rabbah 10:7]

אצבע א-להים הוא: מכה זו אינה על ידי כשפים, מאת המקום :

as the Lord had spoken: “But Pharaoh will not hearken to you” (Exod. 7:4).

כאשר דבר ה': . (ולא ישמע אליכם פרעה (ז ד

16And the Lord said to Moses, "Arise early in the morning and stand before Pharaoh, behold, he is going out to the water, and you shall say to him, 'So said the Lord, "Let My people go out and serve Me.

17For if you do not let My people go, behold, I will incite against you and against your servants and against your people and in your houses a mixture of noxious creatures, and the houses of Egypt will be filled with the mixture of noxious creatures, as well as the land upon which they are.

incite against you: Heb. מַשְׁלִיחַ בְּ, incite against you. Similarly, “and the tooth of beasts I will incite (אִשַׁלַּח) against them” (Deut. 32:24), an expression of inciting, antiziyer in Old French, to incite, to set upon. — [from Jonathan]

a mixture of noxious creatures: [which includes] all species of wild beasts, snakes, and scorpions in a mixture, and they were destroying among them [i.e., among the Egyptians]. There is a reason [given] for this matter in the Aggadah, [i.e.,] for each plague, why this one and why that one. Following a king’s war strategy did He come upon them [the Egyptians], according to the order of a kingdom when it besieges a city. First they [the King’s army] destroy its [the city’s] springs, and then they blow and sound rams’ horns to frighten them and confuse them; thus did the frogs croak and make noise, etc., as is stated in the Midrash of Rabbi Tanchuma (Bo 4).

18And I will separate on that day the land of Goshen, upon which My people stand, that there will be no mixture of noxious creatures there, in order that you know that I am the Lord in the midst of the earth.

And I will separate: Heb. וְהִפְלֵיתִי, and I will set apart. Similarly, “And the Lord will set apart (וְהִפְלָה) ” (Exod. 9:4), and similarly, “it is not separated (נִפְלֵאת) from you” (Deut. 30:11); it is [not] set apart and separated from you. — [from Onkelos]